Vulcanization of dip-molded rubber articles with molten media baths

ABSTRACT

Pore-free rubber articles are prepared by dip-molding in a dipping medium that includes a vulcanizing agent, then by immersing the dip former in a heated liquid bath that is chemically inert. A particularly effective liquid bath is molten inorganic salt. In addition, the tensile properties of an article of vulcanized rubber can be improved to an unusually effective degree by immersing the already vulcanized article in a solution of a vulcanizing agent to cause the rubber of the article to absorb or imbibe the vulcanizing agent from the solution, and then immersing the rubber and the imbibed vulcanizing agent in a heated liquid bath to increase the degree of vulcanization.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention lies in the field of rubber articles, particularlythose formed by dip-molding. In particular, this invention addressesmethods of vulcanization of dip-molded rubber articles.

[0003] 2. Background of the Invention

[0004] Natural rubber latex has been extensively used as a material ofconstruction for elastomeric dip-molded medical devices and medicaldevice components. Examples of medical devices and components made fromnatural rubber latex are surgical gloves, examination gloves, fingercots, catheter balloons, uterine thermal ablation balloons, cathetercuffs, condoms, contraceptive diaphragms, indwelling urinary drainagecatheters, and male external urinary drainage catheters. Other exampleswill be apparent to those skilled in medicine and in the manufacture anduse of these and similar medical devices. Dip-molding techniques arealso used in making elastomeric devices non-medical uses. These includetoy balloons, industrial gloves, household gloves, and other similardevices. These devices, both medical and non-medical, can also be formedfrom synthetic rubber latex materials rather than natural rubber. Insome cases, synthetic materials are preferred, for example where naturalrubber is deemed unsuitable for some reason or where the syntheticmaterial offers an advantage.

[0005] In latex dip-molding processes, dip formers are dipped in a latexbath, then withdrawn from the bath, dried in hot air, and vulcanized inhot air. In some cases, the latex is pre-vulcanized, i.e., the rubberparticles in the latex are partially or fully vulcanized prior to thedipping step. A prevulcanized latex produces a film with improved wetand dry gel strengths, and when further vulcanization is performed afterdipping and hot air drying, the tensile properties are improved. Anadvantage of prevulcanization is a reduction in the process time bylessening or eliminating the time required for the post-dipvulcanization. In some dip-molding processes, a chemical coagulant isincluded in the latex or on the dip former, and heat-sensitizedcoagulant dipping methods are applied to produce articles have a greaterfilm thickness. Multiple dips are also used in some processes. Detailsof these and other methods are well known to those skilled in the art oflatex dip molding. Further descriptions of the process and itsvariations are found in Pendle, Dipping with Natural Latex, published byThe Malaysian Rubber Producers' Association (1995).

[0006] Vulcanization performed on the latex film after the dip former isremoved from the bath serves to form covalent bonds both within theindividual rubber particles and between coalesced rubber particles. Aproblem with vulcanization both at this stage and prior to the dip isthat the outer surfaces of the particles have greater exposure to thevulcanizing agents than the particle interiors, resulting in acase-hardening effect and a lack of uniformity in the rubber.

[0007] In dip-molding processes for rubber latices, sulfur is theprimary vulcanizing agent, although various accelerators, activators,sulfur donors, and boosters are frequently included as well. Adescription of prevulcanization methods and formulations for bothnatural and synthetic rubber latices is found in Blackley, D. C.,Polymer Latices: Science and Technology, 2d Edition, Vol. 2, Chapter 13(Chapman and Hall, 1997). Prevulcanization methods performed withoutsulfur are those utilizing free radical crosslinking, which can beachieved by various means, including high energy irradiation in thepresence of a chemical sensitizer. Natural latex prevulcanized in thismanner is referred to as “radiation vulcanized natural rubber latex”(RVNRL). Descriptions of such latices and the vulcanization processesused in their preparation are found in Zin, W. M. B. W., “Semiindustrial scale RVNRL preparation, products manufacturing andproperties,” Radiat. Phys. Chem., 52(1-6), pp. 611-616 (1998).

[0008] Rubber films from RVNRL are produced by simply casting the latexinto films and then drying the films. No vulcanization is done after thefilm is cast, and none can be done unless curative agents aresubsequently added. Films made by this process have tensile strengths ofup to 27.1 megapascals (3930 psi). While this meets the requirements ofmany dip-molded rubber devices, such as surgical gloves for example, thetensile strength of these films is not as high as that achieved in manysulfur-vulcanized films where a post-vulcanization step (after the dipstage) is included. The RVNRL films are also lower in the value of the100% tensile modulus than sulfur-vulcanized films. The RVNRL films alsosuffer from a lack of any means to achieve true particle integration bycovalent bonds. This makes it difficult to form a truly integrated,pore-free latex rubber film from RVNRL. A further disadvantage is theneed for access to an irradiation facility, which may not be in alocation that is convenient to many rubber manufacturers and which addsconsiderably to the cost of manufacture.

[0009] An alternative means of prevulcanization of latex by free radicalcrosslinking is that which involves the use of organic peroxides andhydroperoxides. Latex that is prevulcanized with these materials isreferred to as “peroxide vulcanized natural rubber latex” (PVNRL).Descriptions of such latices and methods for preparing them are found inU.S. Pat. No. 2,868,859, issued Jan. 13, 1959, to G. Stott, entitled“Curing Natural Rubber Latex With a Peroxide.” The process disclosed inthis patent involves superheating natural rubber latex in the presenceof 2% (based on dry rubber weight) ditertiary butyl peroxide in apressure vessel at a temperature of 170° C. for fifteen minutes. Thelatex was then cooled, and the films cast and dried to yield vulcanizedrubber films with a tensile strength as high as 251 kg/cm² (3739 psi).The film was formed simply by drying, with no post-drying vulcanization.Unfortunately, utilization of this process on a commercial scale wouldrequire large and expensive heated pressure vessels, andprevulcanization is a necessary part of the process.

[0010] Latex prevulcanized with a hydroperoxide rather than an organicperoxide is described in U.S. Pat. No. 2,975,151, issued Mar. 14, 1961,to W. S. Ropp, entitled “Vulcanization of Latex With OrganicHydroperoxide.” In this patent, natural rubber latex is prevulcanized bysuperheating under pressure at 250° F. (121° C.) for about one hour withcumene hydroperoxide. The resulting cooled latex is cast into a film,then air dried. The product had a maximum tensile strength of 2775 psi.As in the Stott patent, the utilization of this process on a commercialscale would require large scale heated pressure vessels, and the tensilestrength is not nearly as good as that of a sulfur-vulcanized latex orof the organic peroxide prevulcanized latex of Stott.

[0011] The use of hydrogen peroxide as a prevulcanizing agent with anactivating chemical is disclosed in U.S. Pat. No. 3,755,232, issued Aug.28, 1973, to B. K. Rodaway, entitled “Vulcanization of Latex WithOrganic Hydroperoxide.” The method of this patent is performed at lowertemperatures without the use of pressure vessels. The patent cites anexample in which a natural rubber latex is prevulcanized by this method,cast into a film and dried, to yield a product with a tensile strengthof 124 kg/cm² (1760 psi). Thus, despite its advantages this processproduces latex films of interior strength. The possibility of adding asulfur curative system to the latex after prevulcanization to permitpost-casting vulcanization is suggested, but this would involve the useof sulfur curative chemicals, which peroxide processes are generallyintended to avoid. In further examples, curing of polychloroprene andother synthetic latices is performed with hydrogen peroxide and anactivator, the products in each case having inferior tensile properties.

[0012] Further disclosure of technology forming the background of thepresent invention is found in U.S. Pat. No. 3,892,697, issued Jul. 1,1975, to O. W. Burke, entitled “Preparation of Crosslinked Polymer LatexFrom Aqueous Emulsion of Solvent/Polymer Solution of Precursor LatexParticle Size.” In the process disclosed in this patent, dicumylperoxide is mixed with a synthetic polyisoprene latex under 6000 psipressure, and the mixture is subjected to an unspecified elevatedtemperature for an unstated period of time. There is no disclosure offilm formation.

[0013] Still further methods forming the background of the invention arethose known as “continuous vulcanization in liquid baths” (LCMVulcanization) which are used on extruded rubber profiles. In LCMVulcanization, a solid constant profile shape is extruded, thensubmerged in a hot liquid bath such as molten salt, hot oil, or meltedlead, or in a hot fluid medium such as fluidized sand particles.Essentially all molecular oxygen is excluded from the curingenvironment. The use of the hot liquid bath or fluid medium is toprovide very rapid heat transfer rates to thin-wall extruded rubberprofiles. Descriptions of various LCM Vulcanization methods are found inHoffman, Rubber Technology Handbook, pages 394-398 (Hanser Publishers,1994), and in U.S. Pat. No. 4,981,637, issued Jan. 1, 1991, to M. L.Hyer, entitled “Method of Forming an Improved Wiper Blade.” Thesereferences do not disclose application of the process to dipped films.

[0014] Latex articles formed by dip molding must be pore-free if thepassage of pathogens or other unwanted substances through the articlewalls is to be prevented. Pore-free walls require good integration andadhesion between the rubber particles of the latex. Many attempts havebeen made to achieve this, but it remains a difficult goal. Excessivevulcanization for example tends to inhibit particle integration. Asimple means of determining the extent of prevulcanization is a testknown as the chloroform coagulation test. A description of this test canbe found in The Vanderbilt Latex Handbook, 3d Edition, page 110 (R. T.Vanderbilt Company, Inc., Norwalk, Conn., USA).

[0015] All patents and publications cited in this specification areincorporated herein by reference.

SUMMARY OF THE INVENTION

[0016] It has now been discovered that pore-free rubber articles can beprepared by dip-molding processes by including vulcanizing agent(s) inthe dipping medium and vulcanizing the wet film by immersing the dipformer in a heated liquid bath that is chemically inert. The temperatureof the heated bath will be sufficiently high to cause at least a partialmelting and/or softening of any coalesced rubber particles in the filmwhile vulcanizing the film, and the time needed to effect vulcanizationunder these conditions is considerably less than that typically used forvulcanization in hot air. The resulting film is coherent and essentiallypore-free.

[0017] Another aspect of this invention resides in the discovery thatthe tensile properties of an article of vulcanized rubber can beimproved to an unusually effective degree by immersing the alreadyvulcanized article in a solution of a vulcanizing agent to cause therubber of the article to absorb or imbibe the vulcanizing agent from thesolution, and then immersing the rubber and the imbibed vulcanizingagent in a heated liquid bath that is substantially free of molecularoxygen and chemically inert. After recovery of the article from thebath, the tensile properties are considerably greater than those thatthe product would have if the same amount and type of vulcanizing agentwere included in the original vulcanization.

[0018] This invention is useful in the manufacture of articles of allrubber materials, both natural and synthetic. For certain aspects ofthis invention, notably those that reside in the use of the heatedliquid bath for a single-stage vulcanization after dip-molding, thepreferred rubber materials are those other than cis-1,4-polyisoprene.

[0019] Among the many advantages of this invention is a fastervulcanization rate without the risk of undesirable oxidation of thedipped parts. The invention also offers superior particle integrationand thus more coherent latex films by partially melting the particles asthey are being crosslinked and heating them more thoroughly, whichreduces the tendency of the particles toward case hardening.Prevulcanization, i.e., vulcanization performed on the dipping liquidprior to the dip stage, can be eliminated in many cases, and this offersadvantages for latices that are peroxide cure systems or sulfur curesystems where prevulcanization is used in part to reduce the cure timesand to reduce the quantity of nitrosamines that may be released duringdip molding operations. Alternatively, the postvulcanization of theinvention, referring to its occurrence subsequent to the dip stage, canimprove the tensile properties of the product without the need for theaddition of sulfur-based chemicals. Postvulcanization can also beperformed using a different reaction than that used for theprevulcanization. For example, postvulcanization with the use ofperoxides can be performed on latices that are prevulcanized by sulfur,by peroxide, or by radiation. In systems that are susceptible tonitrosamine formation, the invention reduces or eliminates the amount ofnitrosamines that are formed. When polychloroprene latices, nitrilelatices or mixtures of the two are used, postvulcanization by use oforganic peroxides can be achieved with very small amounts of theperoxides. With peroxide-based and radiation-based vulcanizationsystems, the use of the present invention provides products with a 100%tensile modulus that is higher than has been previously obtained withsuch systems, and yet with no loss of tensile strength. Withsulfur-based systems, the high-temperature, oxygen-free environmenthelps to prevent the degradation that is caused by hydroperoxides thatare typically generated during hot air vulcanization. Such degradationis responsible in part for the aging of latex. Use of the invention innon-sulfur-containing systems such as peroxide-based systems results inproducts with a longer shelf life. Still further, the elimination of theneed for hot air curing and its inherent inefficiencies offersconsiderable savings in energy, since hot media baths are easilyinsulated. Further energy savings are also available whenprevulcanization and maturation are eliminated. The means by which theseand other objects and advantages are achieved, as well as particulars ofthe process and its preferred embodiments, will be evident from thedescription that follows.

DETAILED DESCRIPTION OF THE INVENTION AND SPECIFIC EMBODIMENTS

[0020] The liquid bath in which the dip former and film are immersedsubsequent to the dip stage of the process is a heated liquid thatprovides rapid heat transfer to the film. Further properties of liquidmedia that are most desirable and therefore preferred for this purposeare the lack of a tendency to migrate or diffuse into the film on thedip former (unless the medium itself is a desirable constituent of thefilm), the quality of being stable with respect to the surroundingenvironment (both the atmospheric environment and the rubber-formingmaterial as well as the various species that may be compounded with thematerial), and the quality of remaining liquid at the vulcanizationtemperature. Examples of liquid media that can be used for this purposeare molten inorganic salts, oils, glycols, liquified metals, water, andbrine solutions. Preferred among these are molten inorganic salts,silicone oils, and glycols, and the most preferred are molten inorganicsalts. Examples of suitable molten inorganic salts are nitrates,nitrites, carbonates, sulfates, phosphates, and halides of potassium,sodium and lithium, as well as combinations of salts of this group. Saltcombinations of this type are commercially available from such suppliersas Heatbath Corporation, Detroit, Mich., USA; and Hubbard-Hall Inc.,Inman, S.C., USA. An example of a suitable commercial salt mixture isQUICK CURE 275 of Hubbard-Hall, Inc., the main components of which arepotassium nitrate (approximately 50% by weight), sodium nitrite(approximately 30% by weight), and sodium nitrate (less than 10% byweight), with a molten temperature range of about 315° F. to 650° F.(157° C. to 343° C.). Other examples are PARCURE 275 and PARCURE 300 ofHeatbath Corporation.

[0021] The heated liquid medium bath is preferably used at a temperaturethat significantly exceeds the temperatures used in hot airvulcanization methods of the prior art, but not so high as to have anadverse effect on the stability of the rubber being vulcanized. When therubber is natural rubber, for example, it is best not to exceed 450° F.(232° C.), and in the case of styrene butadiene rubber orpolychloroprene latex, it is best not to exceed 575° F. (302° C.). Apreferred temperature range for the full scope of this invention isabout 100° C. to about 350° C. For polychloroprene and styrene-butadienerubber, a preferred temperature range is about 150° C. to about 300° C.,while for natural rubber a preferred temperature range is from about150° C. to about 235° C. The choice of operating temperature andexposure time will be subject to considerations both of achieving arapid cure and of maintaining an economic use of energy and otherprocess costs. Other considerations may be present with particular typesof rubber and particular curing systems. In organic peroxide curingsystems, for example, the preferred temperature and time will be thosethat result in cleavage of essentially all of the peroxide present. Thisis generally achieved in six to eight half-lives. In sulfur-based curingsystems, the avoidance of reversion and toxicity are oftenconsiderations. In all cases, however, the time necessary for fullcuring is much less than that required in hot air curing processes ofthe prior art. A presently preferred cure condition is nine minutes at350° F. (177° C.).

[0022] This invention is applicable to a wide range of rubber and rubbersubstitute compositions, including both latices and organic solutions.

[0023] Of the latices, the one most commonly used is natural rubber.Natural rubber can be obtained from several sources, including Heveabrasiliensis, Parthenum argentatum (commonly known as “guayule”), andFicus elastica rubber trees. Methods for obtaining natural rubberlatices from non-Hevea sources are described in U.S. Pat. No. 5,580,942,issued Dec. 3, 1996 to Cornish (“Hypoallergenic Natural Rubber ProductsFrom Parthenum Argentatum (Gray) and Other Non-Hevea BrasiliensisSpecies”). Natural rubber latex is available in several grades,including high ammonia latex, low ammonia latex, and others. All suchvarieties are suitable for use in the present invention. This inventionalso extends to natural rubber latices that have been processed toreduce the amount of proteins present in the latices. Some of theseprocesses include centrifuging to separate and remove water, and othersinclude double centrifuging, in which an initial centrifuging isfollowed by the addition of water and a second centrifuging. Still otherprocesses involve the use of enzymes to digest the proteins.Descriptions of enzyme methods are found in U.S. Pat. No. 5,610,212(“Means for Mechanically Stabilizing Deproteinized Natural RubberLatex,” Mar. 11, 1997), U.S. Pat. No. 5,569,740 (“Deproteinized NaturalRubber Latex and Its Production Process,” Oct. 29, 1996), and U.S. Pat.No. 5,585,459 (“Process for Producing Raw Rubber,” Dec. 17, 1996), toTanaka et al. An example of a commercially available deproteinizedrubber latex is ALLOTEX, obtainable from Tillotson HealthcareCorporation, Rochester, N.H., USA.

[0024] Synthetic rubber latices in general are likewise usable in thepractice of this invention. Examples are ethylene-propylene-dieneterpolymer, styrene isoprene rubber, styrene butadiene rubber, styreneisoprene butadiene rubber, polybutadiene rubber, polychloroprene,nitrile rubber, styrene block copolymers, and butyl rubber. An exampleof a polychloroprene latex is NEOPRENE 750, available from E. I. DuPontde Nemours, Inc. Wilmington, Del., USA, and an example of a nitrilelatex is NITRILE LATEX #O17071, available from Heveatex Corporation,Fall River, Mass., USA. Mixtures of latices can also be used. Some ofthese mixtures are described in U.S. Pat. No. 3,626,052, issued Dec. 7,1971, to Sisco et al., entitled “Polyisoprene-Neoprene MeteorologicalBalloons,” where polychloroprene latex is mixed with polyisoprene latexto produce meteorological balloons.

[0025] This invention also extends to polymer dispersions that are usedin a manner similar to rubber latices. One example is an aqueousdispersion of a polyurethane thermoplastic elastomer. A commerciallyavailable dispersion of this type is INTACTA, available from The DowChemical Company, Midland, Mich., USA. Polymer dispersions such as thislack carbon-carbon double bonds and hence are not susceptible tosulfur-based crosslinking. For these dispersions, embodiments of thepresent invention that use curing systems other than those that aresulfur-based can be used. Polyurethane products such as medicalexamination gloves that are formed by the process of this inventionexhibit increased resistance to solvents.

[0026] In addition to latices and polymer dispersions, the presentinvention also applies to organic solutions. The organic solvents usedin forming these solutions are any solvents that are inert to therubber, rubber substitute or polymer, and that are readily removablefrom the dip-molded film by evaporation. The solvent is preferably analiphatic hydrocarbon, saturated or unsaturated, linear, branched orcyclic, or ethers, esters, alcohols or amines. Typical solvents arealiphatic hydrocarbons containing 5 to 8 carbon atoms, such as pentane,pentene, hexane, heptane, cyclohexane, and cyclopentane, andheterocyclic compounds such as tetrahydrofuran.

[0027] A wide variety of vulcanizing agents can be used in the practiceof this invention. Useful vulcanizing agents include organic peroxides,sulfur-containing compounds, selenium-containing compounds, andtellurium-containing compounds. Organic peroxides, for example, may beused singly or in combination, and the most common types are dialkylperoxides, peroxyketals, and dialkyl peroxides. Preferred organicperoxides are the dialkyl peroxides, particularly dicumyl peroxide,available from Hercules Incorporated, Wilmington, Del., USA, as DICUP R.Other useful dialkyl peroxides are2,5-dimethyl-di-(t-butylperoxy)hexane, di-t-butylperoxide,t-butylcumyl-peroxide, bis(t-butylperoxyisopropyl)benzene, butyl4,4-bis(t-butylperoxy)valerate,2,5-bis(t-butylperoxy)-2,5-dimethylhexane,2,5-bis(t-butylperoxy)-2,5-dimethyl-3-hexyne, t-butyl 3-isopropenylcumylperoxide, bis(3-isopropenylcumyl) peroxide,1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,t-butylperoxybenzoate, and bis(2,4-dichlorobenzoyl) peroxide.

[0028] Coagents and other additives are often used in conjunction withthe organic peroxides to achieve products having particular properties.Certain coagents also add to the crosslinking efficiency of theperoxides by causing a single peroxide radical to produce more than onecarbon-carbon crosslink. Coagents can also be integrated into thepolymer network by covalent bonds to enhance certain properties of thepolymer, such as elongation and tear strength. Some of these coagentsare based on acrylate and methacrylate chemistry. All however aresuitable for inclusion in the methods and products of the presentinvention. Examples of suitable coagents are SARET 516, SARET 517, SARET521, and SARET 634, available from Sartomer Company, Inc., Exton, Pa.,USA. These coagents are multifunctional salts of acrylic and methacrylicacids. Of this group of coagents, SARET 634 (whose primary ingredient iszinc dimethacrylate) and SARET 521 (whose primary ingredients aredifunctional acrylate esters) are the most preferred. Trimethylolpropanetrimethacrylate another example. A more extensive description of suchcoagents is found in U.S. Pat. No. 3,751,878, issued Aug. 7, 1973 toCowperthwaite et al., entitled “Inhibiting Prevulcanization of RubberWith Polyfunctional Methacrylate Monomers as Cross-Linking Coagents withPeroxides,” and U.S. Pat. No. 5,310,811, issued May 10, 1994 to Cottmanet al., entitled “Free Radical Cured Rubber Employing Acrylate orMethacrylate Esters of Hydroxybenzene and Hydroxynaphthalene Compoundsas Co-Curing Agents.”

[0029] Free-radical vulcanizing agents other than peroxides aredisclosed in U.S. Pat. No. 3,892,697, referenced above.

[0030] Sulfur-based vulcanization systems include both smallsulfur-containing molecules and sulfur-containing polymers. Examples ofsulfur-based vulcanization chemicals are:

[0031] mercaptothiazoles, for example 2-mercaptobenzothiazole and itssalts, notably its zinc salt

[0032] thiuram sulfides and disulfides, for example tetraethylthiurammonosulfide, tetrabutylthiuram monosulfide, tetramethylthiuramdisulfide, and tetraethylthiuram disulfide,

[0033] guanidines

[0034] thiourea and substituted thioureas

[0035] thiocarbanilides and substituted thiocarbanilides, for exampleo-dimethyl-thiocarbanilide and its isomers and alkyl homologs

[0036] zinc alkyl dithiocarbamates, for example zinc dimethyldithiocarbamate, and accelerators containing these materials

[0037] sodium or potassium dimethyl dithiocarbamate

[0038] selenium dialkyl dithiocarbamates, for example seleniumdiethyldithiocarbamate

[0039] 2-benzothiazyl-N,N-diethylthiocarbamyl sulfide

[0040] xanthates such as dibutyl xanthogen disulfide and xanthogenpolysulfide

[0041] alkyl phenol sulfides

[0042] dipentamethylene tetrasulfide

[0043] sulfur-containing polymers such as Thiokol VA-3

[0044] 4,4-dithiomorpholine

[0045] miscellaneous disulfides such as bensothiazyl disulfide andbis(dimethylthiocarbamoyl) disulfide

[0046] When the dip-molded articles of this invention are intended foruse in contact with human skin, the preferred compounding ingredientsare those that produce films that are biocompatible. Examples ofcompounding ingredients that serve this purpose for sulfur-vulcanizedsystems are xanthogen compounds such as diisopropyl xanthogenpolysulfide, dibenzyldithiocarbamate, and higher alkyl zincdithiocarbamates. For peroxide vulcanized systems, the preferredcompounding ingredient is dicumyl peroxide.

[0047] Reinforcing agents and other additives are also included in someembodiments of the invention. Examples of reinforcing agents are fumedsilica, carbon black, and chopped fibers. The use of cut fibers forexample to improve the tear strength of medical gloves is disclosed inU.S. Pat. No. 6,021,524, issued Feb. 8, 2000, to Wu et al., entitled“Cut Resistant Polymeric Films,” and the use of fumed silica to improvethe tear strength of dipped films is disclosed in U.S. Pat. No.5,872,173, issued Feb. 16, 1999, to Anand, entitled “Synthetic LatexCompositions and Articles Produced Therefrom.” Antioxidants andantiozonants may also be included to protect against environmentalaging. Pigments and dyes may also be included, as may any of the otheradditives known to those skilled in the art of the formulation andmanufacture of rubber devices.

[0048] An illustrative procedure for latex dip molding and curing inaccordance with the present invention is as follows:

[0049] 1. Either a natural rubber or a synthetic rubber latex iscompounded with vulcanizing agent(s) and possibly an antioxidant, astabilizer or both. If organic peroxide vulcanization is used, it willoften be sufficient to simply add to the latex a dispersion thatcontains an organic peroxide.

[0050] 2. Prevulcanization of the latex at this stage is optional andnot required for all embodiments of this invention. When used,prevulcanization can improve the wet gel strength.

[0051] 3. A dip former is optionally coated with a chemical coagulant bydipping the former into a bath of a coagulant-containing liquid, thenwithdrawing the former and drying it.

[0052] 4. The dip former, with or without the coagulant coating, isdipped in a bath filled with the compounded latex.

[0053] 5. The dip former is slowly withdrawn from the bath. If theformer had a coagulant coating, it now has a wet latex gel on itssurface. If no coagulant coating was applied, the former will have aliquid latex film on its surface.

[0054] 6. Excess water in the latex film on the dip former surface isremoved, generally by evaporation in a hot air convection oven witheither sweep gas or a partial vacuum. The process can be supplementedwith infrared, microwave, or radiofrequency radiation, or any other typeof energy to expedite the evaporation. Vacuum drying is advantageoussince it avoids the need for exposure of the dried latex to air at anelevated temperature prior to vulcanization.

[0055] 7. The latex is cured by immersion of the dip former into theheated liquid media bath for sufficient time to cure the latex.

[0056] 8. The dip former with the cured latex film is withdrawn from theheated medium bath and cooled either in air or in a stream of water.Water may be used to rinse off any excess solidified heat transfermedium such as solidified salt.

[0057] 9. The vulcanized latex article is manually or mechanicallystripped from the dip former.

[0058] An illustrative procedure for solvent dip molding and curing inaccordance with the present invention is as follows:

[0059] 1. Solid granules of synthetic or natural rubber elastomer aredissolved in a suitable solvent to form a cement. Suitable compoundingagents are dispersed or dissolved in the cement. Compounding agentssimilar to those used in the latex processes, including organicperoxides, can be used.

[0060] 2. No prevulcanization is necessary, as all compounding gents areuniformly dispersed in the cement. The cement is placed in a dip tank,and a dip former is dipped in the cement.

[0061] 3. The dip former is slowly withdrawn from the dip tank to leavea film of the cement over the surface of the dip former.

[0062] 4. Solvent is evaporated from the dip former to leave a uniformpolymer film on the surface. Removal of the solvent can be achieved byambient or hot air drying.

[0063] 5. The polymer film is cured by immersion of the dip former in aheated liquid medium bath. After a suitable period of time, the dipformer is withdrawn from the bath and cooled in air or a stream ofwater.

[0064] 6. The dip former is then soaked in water to help break theadhesion between the film and the dip former.

[0065] 7. The vulcanized latex article is manually or mechanicallystripped from the dip former.

[0066] While the present invention virtually eliminates the need forprevulcanization and maturation of the compounded latex or solution,prevulcanization is useful with latices that would otherwise have anexceptionally low wet or dry gel strength. Prevulcanization can be doneby any conventional method. Such methods include, but are not limitedto, sulfur prevulcanization, peroxide prevulcanization, andprevulcanization by high energy irradiation, all of which may beperformed as they are in the prior art. Good wet gel strength is usefulin preventing cracks from forming in the film as the film is beingdried. In the case of natural rubber, both wet and dry gel strengths aregenerally adequate without prevulcanization. The gel strengths of somesynthetic latices are lower, however, and prevulcanization may improvethe processing, but is not essential. Prevulcanization by high energyirradiation can also serve to reduce the amount of vulcanizationchemicals needed and hence the levels of undesirable residual chemicalsin the final product.

[0067] It is often useful to determine the extent to which a dipped filmor article has been vulcanized. A commonly used method is to cut out acircular disk of the cured film and measure the change in diameter uponimmersion of the disk in a solvent bath. A detailed explanation of thistest and its use with polyisoprene latex is found in U.S. Pat. No.3,215,649, issued Nov. 2, 1965, to Preiss et al., entitled “SyntheticLatex.” Similar test methods are available for other types of vulcanizedpolymers, and are well known to those skilled in the art.

[0068] After the dip-molded part is vulcanized, further vulcanizationcan be performed as an optional means of further improving theproperties of the product. A preferred method is to imbibe thevulcanized film with a further amount of vulcanizing agent(s), followedby a second heat treatment in a hot liquid bath. The vulcanizing agentmay be the same or different than that used in the first stage(immediately following the dip-molding stage). Likewise, the hot liquidbath may be the same or different than that used earlier.

[0069] For films vulcanized with dicumyl peroxide, for example, thecured rubber film can be immersed in a solution of dicumyl peroxide in asolvent such as n-pentane, n-hexane, toluene, or ethyl acetate. Theperoxide solution significantly swells the film, thereby causing thedicumyl peroxide and solvent to uniformly penetrate the cured film. Thefilm is then withdrawn from the solution and the solvent evaporated,leaving a predictable amount of dicumyl peroxide in the film. The filmis then immersed in a hot liquid bath as before for an appropriateperiod of time, which may be the same period of time used after theinitial dip in the latex. The film is then removed and rinsed in water.Other vulcanizing agents or combinations of vulcanizing agents can besubstituted to similar effect.

[0070] The physical properties of crosslinked articles that arevulcanized in this two-step postvulcanization process are differentfrom, and frequently better than, those of crosslinked articles in whichonly a single postvulcanization has been performed. This secondpostvulcanization thus permits a reworking of or an enhancement of theproperties of films that have already been vulcanized. This isparticularly useful, for example, in the case of right-heart catheterballoons, where the second postvulcanization can achieve significantlyhigher levels of air inflation and burst pressures. Returning to thedicumyl peroxide example, a typical range of dicumyl peroxide for a highquality right heart catheter is about 1 to about 1.5 phr (parts hundredratio, or parts per hundred weight of dry rubber). Of this, 0.2 to 0.5extra phr of dicumyl peroxide can be imbibed with a subsequent heatingstep to achieve a significant improvement in the air inflation and burstproperties.

[0071] The following examples are offered for purposes of illustration,and are not intended to limit the scope of the invention. All patentsand publications cited in these examples are hereby incorporated hereinby reference.

EXAMPLE 1—COMPARATIVE

[0072] This comparative example demonstrates the degradation ofproperties that occurs when latex films are dip molded from organicperoxide-containing latices and then vulcanized in an oxygen-bearingenvironment.

[0073] A 40% solids dicumyl peroxide emulsion was prepared by combiningthe following ingredients: Dicumyl peroxide 100 parts by weight Toluene35 parts by weight Oleic acid 5.6 parts by weight De-ionized water 101parts by weight 30% Aqueous KOH 2.6 parts by weight

[0074] Natural rubber latex at a concentration of 60 weight percentsolids was used, supplied by Diversified Compounders, Inc., Los Angeles,Calif., USA. An aqueous coagulant containing approximately 35% calciumnitrate, 0.5% Igepal CO-630 surfactant (Rhone-Poulenc, Cranbury, N.J.,USA), and 64.5% deonized water (all by weight) was also used. Clearglass tubes 32 mm in diameter were used as dipping formers.

[0075] To 1 kg of the natural latex was added 21 g of the dicumylperoxide emulsion. The resulting composition was mixed under mediumshear for thirty minutes with a laboratory mixer. The mixture was thenrolled for thirty minutes on a laboratory roll mill, then degassed forten minutes at 0.3 atmosphere absolute pressure. This yieldedapproximately 1 liter of natural rubber latex formulated with 1.4 phrdicumyl peroxide.

[0076] The glass formers were dipped in the coagulant solution, allowedto dwell for five seconds, withdrawn, and then, without drying, weredipped in the formulated latex. The formers were allowed to dwell in thelatex for a period of five seconds, then slowly withdrawn. The formerswere then dried in a hot air oven at 60° C. for sixty minutes. Afterdrying, the formers were placed in a hot air curing oven at 110° C. Oneformer was withdrawn every ten minutes and the latex film examined.Observations at ten-minute intervals over a fifty-minute period werethus made, and the results are listed in Table I. TABLE I ComparativeExample: Appearance and Physical Properties of Dipped Films vs. TimeSpent in Hot Air Curing Oven at 110° C. Time (minutes) Observations 10Clear, non-tacky film; good green tensile strength 20 Clear, non-tackyfilm of darker brown shade; green tensile strength lower than that at 10minutes 30 Film very dark in color although still non-tacky; tensilestrength very low, crumbled on touch 40 Film very dark in color andslightly tacky; no tensile strength 50 Film very dark in color and tacky(more than slightly); no tensile strength

[0077] The observations in Table I indicate that the dipped latex filmsprepared from organic peroxide formulated latices cannot be successfullycured in a hot air oven due to the interaction with the oxygen in thecuring environment.

EXAMPLE 2—PROCESS ACCORDING TO THE INVENTION Natural Rubber Latex

[0078] This example illustrates the process of the present inventionusing the same materials as those of Example 1 but substituting a moltensalt bath cure for the hot air cure. A coagulant solution in ethanol wasused, containing approximately 20% calcium nitrate, and 0.5% IgepalCO-630, all by weight, the balance denatured ethanol.

[0079] To 1 kg of natural rubber latex was added 19.5 g of the dicumylperoxide emulsion, and the resulting composition was mixed under mediumshear for thirty minutes on a laboratory mixer. In addition, fumedsilica was added at 2 phr in the form of a 15% (by weight) aqueousdispersion (CABO GUARD T-122) supplied by Cabot Corporation, BostonMass., USA. After thirty minutes of mixing, the solution was rolled forthirty minutes on a laboratory roll mill, then degassed for ten minutesat 0.3 atmosphere absolute pressure. This yielded approximately 1 literof natural rubber latex formulated with 1.3 phr dicumyl peroxide.

[0080] The glass former was dipped into the coagulant solution, thendried for five minutes at 40° C., then slowly dipped into the formulatedlatex where the former was allowed to dwell for five seconds. The formerwas then slowly withdrawn and dried at 60° C. for sixty minutes. Oncedried, the former and its adherent film were immersed in a molten saltbath for nine minutes at 350° F. (177° C.). The film was then removedfrom the salt bath, rinsed, stripped and readied for tensile testing.The film appeared translucent-to-clear and slightly amber in color. Itswas more transparent than many sulfur-vulcanized rubber films.

[0081] A standard condom ring tensile specimen was prepared and testedin accordance with ASTM specification D3492. The tensile values obtainedare listed in Table II: TABLE II Invention Example—Natural Rubber:Tensile Modulus % Elongation Modulus (psi)  50  78 100 114 200 178 300264 400 313 500 1292  600 2954  700 not recorded At break (ultimate5659  tensile strength)

[0082] The ultimate elongation of the test specimen was 706 percent.

[0083] These results show that the tensile strength of this material isoutstanding when compared with previously published values for naturalrubber latex vulcanized by any known means. Comparison of these resultswith ASTM standard D3577-98 (“Standard Specification for Rubber SurgicalGloves”) and ASTM standard D3492 (“Standard Specification for RubberSurgical Gloves”) indicates that the film produced in this example canmeet the necessary tensile strength requirements for both surgicalgloves and condoms.

EXAMPLE 3—PROCESS ACCORDING TO THE INVENTION Polychloroprene

[0084] This example illustrates the process of the present invention asapplied to polychloroprene, using procedures similar to those of thepreceding examples. The polychloroprene was a latex containing 60 weightpercent solids, supplied by DuPont-Dow Elastomers, LLC, Wilmington,Del., USA and is sold commercially as NEOPRENE 750.

[0085] A dicumyl peroxide emulsion as in Example 1 was added to thelatex to attain a formulated latex containing 0.1 phr dicumyl peroxide.Also added to the latex was fumed silica (reinforcing agent), added as a15 weight percent aqueous dispersion (supplied by Cabot Corporation,Boston, Mass., USA, as CABO GUARD T-22) to achieve a level of 3 phrfumed silica.

[0086] The glass former was first dipped into an aqueous coagulantsolution, which contained 35% calcium nitrate, 0.5% IGEPAL CO-630surfactant, both by weight, the balance dionized water, then allowed todry. The former was then dipped in the compounded latex and allowed todwell in the latex for five seconds, then slowly withdrawn and dried at60° C. for sixty minutes. After drying, the former with latex film wereimmersed in a molten salt bath having the same composition as the bathsused in the preceding examples, for nine minutes at 350° F. (177° C.).The former and film were then withdrawn from the salt bath, rinsed,stripped, and readied for tensile testing. The resultant latex film wastransparent and amber in color.

[0087] Tensile measurements were made in accordance with ASTMspecification D3492, using three tensile rings, to yield the tensilevalues that are listed in Table III. TABLE III InventionExample—Polychloroprene: Tensile Modulus Tensile Tensile Tensile MedianModulus— Modulus— Modulus— Tensile Percent Ring 1 Ring 2 Ring 3 ModulusElongation (psi) (psi) (psi) (psi)  50  87  78  82  82 100 116 108 110110 200 150 145 146 146 300 192 189 188 189 400 278 279 271 278 500 519543 528 528 600 1036  1014  1076  1036  700 2022  1979  2125  2022  AtBreak 3621  3339  3254  3339  (Ultimate Tensile Strength) Ultimate   787%    764%    763% Elongation at Break

[0088] These tensile values are more than sufficient to pass the ASTMstandard D-3577-98 for synthetic rubber surgical gloves.

EXAMPLE 4—PROCESS ACCORDING TO THE INVENTION Polyurethane

[0089] This example illustrates the process of the present invention asapplied to polyurethane, and specifically, in the modification ofthermoplastic polyurethane films after the films have been formed.

[0090] Two solvent dip molding solutions were prepared. The firstconsisted of 15 weight percent thermoplastic polyurethane (MORTHANEPS49, Rohm and Haas Company, Chicago Heights, Ill., USA) and 85 weightpercent tetrahydrofuran. A control film (in the form of a condom) wasprepared by dipping the form into an organic solution, as described inU.S. Pat. No. 4,954,309, issued Sep. 4, 1990, to McGlothlin et al.,entiled “Method of Forming a Polymeric Casing With Textured Surface.”After drying, the polyurethane condom thus formed was stripped from theformer. The second dip molding solution was formed by adding 0.5 phrdicumyl peroxide to the first solution, and a second dip-molded condomwas prepared in a manner essentially identical to the first, except thatthe dipped and dried condom was then immersed for nine minutes in amolten salt bath (identical to those used in the preceding examples) at350° F. (177° C.).

[0091] Portions of both the control condom and the test condom weresubjected to a solvent resistance test. According to this test, bothfilms were immersed in tetrahydrofuran. The control film dissolvedentirely when immersed in the tetrahydrofuran, while the second, whichhad been crosslinked by the dicumyl peroxide treatment, did not dissolvebut instead swelled significantly. This test illustrates the improvementin properties of dip-molded articles made of polyurethane (asrepresentative of thermoplastic elastomers in general) as a result ofthe process of the present invention.

EXAMPLE 5—PROCESS ACCORDING TO THE INVENTION Prevulcanized NaturalRubber Latex

[0092] This example illustrates the process of the present inventionapplied to two prevulcanized natural rubber latices, one by sulfur andthe other by radiation. The sulfur-prevulcanized latex was 60% solidsREVULTEX HLA-21 from Revertex Americas, St. Louis, Mo., USA. Theradiation-prevulcanized latex was obtained from Guthrie Latex, Inc.Tucson, Ariz., and simply sold as “RVNRL.” Both latices are noted fortheir low levels of residual chemicals and hence their low toxicityprofiles. Because of the low toxicity profiles, the tensile strengths ofthese materials are lower than those of many other natural rubberlatices. Standard clear-glass condom formers, 32 mm in diameter, as usedin all preceding examples were used as dip formers.

[0093] Four compounded latices were used, as follows:

[0094] 1. REVULTEX HLA-21 (sulfur-prevulcanized latex) as supplied byRevertex Americas.

[0095] 2. REVULTEX HLA-21 (sulfur-prevulcanized latex) as supplied byRevertex Americas, supplemented with dicumyl peroxide to 1.0 phr.

[0096] 3. RVNRL as supplied by Guthrie Latex, Inc.

[0097] 4. RVNRL as supplied by Guthrie Latex, Inc., supplemented withdicumyl peroxide to 1.0 phr.

[0098] One condom was formed from each of these three latices, using thecoagulant solution and the dipping and drying procedures of Example 3.All were then dried for sixty minutes at 60° C. The condoms formed fromlatices that did not contain dicumyl peroxide were further dried for 45minutes at 150° F. (66° C.) in a hot air oven, powdered, stripped, andset aside. The condoms formed from latices that did contain dicumylperoxide were further processed by immersion in a molten salt bath ofthe same description as those used in the preceding examples, for nineminutes at 350° F. (177° C.). All four condoms were rinsed, powdered,and stripped.

[0099] Tensile values were obtained for all four condoms, using thestandard procedures described in the preceding examples. The results arelisted in Table IV. TABLE IV Invention Example—Prevulcanized NaturalRubber: Tensile Moduli Sulfur- Radiation- Sulfur- Radiation-Prevulcanized Prevulcanized Pre- Pre- Latex with Latex with Percentvulcanized vulcanized Peroxide Post- Peroxide Post- Elongation Latex(psi) Latex (psi) Cure (psi) Cure (psi)  50  58  45  63  71 100  78  61 97 104 200 115  85 149 161 300 159 112 204 226 400 302 180 321 371 500711 437 955 910 600 1546  1012  2498  2222  700 2877  1898  n/a n/a AtBreak 3384  2638  4058  4741  (Ultimate Tensile Strength) Ultimate   732%    756%    665%    710% Elongation at Break

[0100] These results show that the properties of the dip-molded condomsof both methods of prevulcanization, sulfur-based and radiation, areenhanced by postvulcanization in accordance with the present invention.

EXAMPLE 6—PROCESS ACCORDING TO THE INVENTION Addition of VulcanizingAgent by Imbibition for Secondary Postvulcanization

[0101] This example illustrates that aspect of the present invention inwhich a vulcanized and fully formed dip-molded article is given asecondary postvulcanization by first immersing the article in a solutionof a vulcanizing agent to absorb the agent from the solution and thenre-curing the article following the absorption. The rubber material usedin this example was synthetic polyisoprene rubber, supplied as a 10%solids solution in n-hexane. The polyisoprene was NATSYN 2200, fromGoodyear Tire and Rubber Company, Akron, Ohio, USA, and was dissolved inthe hexane by agitating with a medium-shear laboratory mixer. Theresulting solution was split into two batches, and the first wassupplemented by the addition of dicumyl peroxide to 1.5 phr while thesecond was supplemented by the addition of dicumyl peroxide to 2.0 phr.Stainless steel dipping mandrels with outside diameters of 0.091 inch(0.23 cm) were dipped in the solutions, withdrawn, air dried andre-dipped in a sequence that was repeated approximately seven times tobuild up a single wall balloon thickness of approximately 0.010 inch(0.0254 cm). After thorough drying in a warm air oven to removeessentially all of the solvent, the portions of the dipping formers thatwere coated with the dried mixture of polyisoprene and dicumyl peroxidewere immersed in a hot molten salt bath (the same as that used in thepreceding examples) for nine minutes at 350° F. (177C.). The resultingballoons were rinsed in water, powdered with corn starch, and removedfrom the dipping formers. Each balloon was then cut into segmentsapproximately 1 cm in length to form right heart catheter balloons.

[0102] Six of the balloons formed from the 1.5 phr dicumyl peroxidedipping solution were immersed for thirty minutes in an imbibingsolution consisting of dicumyl peroxide dissolved in ethyl acetate, thesolution having a sufficient concentration and volume to raise thedicumyl peroxide content of the balloons by 0.5 phr. The balloons werethen removed from the solution and thoroughly air-dried in a warm airoven to remove essentially all ethyl acetate. The balloons were thenimmersed in a molten salt bath (as described in the preceding examples)for nine minutes at 350° F. (177° C.). The balloons were then removed,rinsed in water, dried, and powdered with corn starch.

[0103] A representative balloon from each treatment group was mounted onan inflation test fixture, and subjected to an air pressure burst testto determine the pressure needed to rupture the balloon upon inflation.The results are shown in Table V. TABLE V Invention Example—Single-Stagevs. Dual-Stage Postvulcanization Burst Pressures Number of Post- DicumylBurst Vulcanization Peroxide Pressure Stages Level (phr) (psig) one 1.513.2 one 2.0 15.4 two 1.5 + 0.5 = 2.0 28  

[0104] These results demonstrate that an unexpected improvement inphysical properties is achieved by a two-stage postvulcanizationachieved by the imbibition of a vulcanizing agent by an already-formeddip-molded rubber article, followed by vulcanization in a hot liquidbath, as compared to a single-stage postvulcanization at the same levelof vulcanizing agent.

EXAMPLE 7—COMPARISON USING NATURAL RUBBER LATEX Hot Liquid Medium CureAccording to the Invention vs. Hot Air Cure of Prior Art

[0105] This example demonstrates the improvement offered by the presentinvention relative to the hot air curing method of the prior art.Natural rubber latex supplemented with a sulfur-based curing system wasused in this comparison.

[0106] Natural rubber latex (60% solids) was supplemented with a curingsystem bearing the name OCTACURE 590 (Tiarco Chemical, Dalton, Ga., USA)in an amount which, according to the supplier, results in a compoundedlatex containing 2 phr zinc oxide, 1.65 phr sulfur, 0.5 phrzinc-2-mercaptobenzothiazole, and 0.75 phr of an unspecifiedantioxidant. The latex was degassed, and two condoms were prepared fromthe latex in the manner described in Example 2, involving the use of thecoagulant described in that example. One of the condoms while still onthe former was then vulcanized in hot air for 45 minutes at 100° C., andthen for an additional sixty minutes at 100° C. The second condom, alsowhile still on the former, was dried for 45 minutes at 100° C., thenimmersed in a molten salt bath of the same description as those used inthe preceding examples for nine minutes at 350° F. (177° C.).

[0107] Tensile test were performed on both condoms in the mannerdescribed in Example 2. The test results are listed in Table VI. TABLEVI Comparison—Hot Air vs. Molten Salt Bath Postvulcanization TensileModuli Tensile Modulus Tensile Modulus Percent After Hot Air AfterMolten Salt Elongation Cure (psi) Cure (psi)  50  42  61 100  55  86 200 67 124 300  82 166 400 115 223 500 210 389 600 425 822 700 767 1588 800 (n/a 2802  At Break 922 3550  (Ultimate Tensile Strength) Ultimate   736%    848% Elongation

[0108] These data demonstrate that the present invention is applicableto natural latex rubber without the need for prevulcanization, and italso shows that the process of the invention produces a product whosetensile properties greatly exceed those of corresponding productsprepared by processes of the prior art.

EXAMPLE 8—APPLICATION OF THE INVENTION TO LATEX MIXTURES

[0109] This example demonstrates the application of the process of theinvention to a mixture of latices.

[0110] A mixture was prepared by combining equal parts by weight ofShell IR-307 synthetic polyisoprene latex and NEOPRENE 750polychloroprene latex. The dicumyl peroxide dispersion described inExample 1 was added to achieve a latex containing 0.7 phr dicumylperoxide. One condom was produced from this latex, using the methoddescribed in Example 2, then immersed in a molten salt bath of the samedescription as those used in the preceding examples for nine minutes at350° F. (177° C.), then rinsed and powdered. The condom was opaque andamber in color. Test specimens were prepared and tensile modulusmeasurements were taken as in the preceding examples. The results arelisted in Table VII. TABLE VII Invention Example—Mixed Latices: TensileModuli Tensile Modulus Percent After Molten Salt Elongation Cure (psi) 50  60 100  91 200 139 300 200 400 429 500 1076  600 2254  At Break(Ultimate 2550  Tensile Strength) Ultimate    619% Elongation

[0111] The foregoing is offered primarily for purposes of illustration.It will be readily apparent to those skilled in the art that thematerials and their proportions, as well as the operating conditions,procedural steps and other parameters of the inventions described hereinmay be further modified or substituted in various ways without departingfrom the spirit and scope of the invention.

What is claimed is:
 1. A method for the preparation of a substantiallypore-free article of rubber other than cis-1,4-polyisoprene, said methodcomprising: (a) dipping a forming member in a liquid medium comprising(i) a rubber-forming substance other than cis-1,4-polyisoprene and (ii)a vulcanizing agent, said forming member having an outer surface with acontour complementary to that of said article; (b) withdrawing saidforming member from said liquid medium in such a manner as to leave afilm of said liquid medium over said outer surface; (c) immersing saidforming member with said liquid film thereon in a chemically inertliquid bath at a temperature and for a period of time sufficient tocause vulcanization of said rubber-forming substance by said vulcanizingagent; and (d) withdrawing said forming member from said liquid bath andseparating said substantially pore-free rubber article from said formingmember.
 2. A method in accordance with claim 1 in which saidrubber-forming substance is a member selected from the group consistingof natural rubber, polychloroprene, nitrile rubber, polyurethane,styrene block polymer, and butyl rubber.
 3. A method in accordance withclaim 1 in which said rubber-forming substance is a polyurethanethermoplastic elastomer.
 4. A method in accordance with claim 1 in whichsaid rubber-forming substance is a polyurethane thermoplastic elastomerand said liquid medium is an aqueous dispersion.
 5. A method inaccordance with claim 1 in which said rubber-forming substance is athermoplastic styrenic block copolymer.
 6. A method in accordance withclaim 1 in which said rubber-forming substance is a thermoplasticstyrenic block copolymer and said liquid medium is an aqueousdispersion.
 7. A method in accordance with claim 1 in which said liquidmedium of step (a) is a latex.
 8. A method in accordance with claim 1 inwhich said liquid medium of step (a) is a solution.
 9. A method inaccordance with claim 1 in which said rubber-forming substance is amember selected from the group consisting of natural rubber andpolychloroprene.
 10. A method in accordance with claim 1 in which saidliquid bath of step (c) is a member selected from the group consistingof molten inorganic salts, oils, glycols, liquified metals, water, andbrine solutions.
 11. A method in accordance with claim 1 in which saidliquid bath of step (c) is a member selected from the group consistingof molten inorganic salts, silicone oils, and glycols.
 12. A method inaccordance with claim 1 in which said liquid bath of step (c) is amember selected from the group consisting of molten inorganic salts andmixtures thereof.
 13. A method in accordance with claim 12 in which saidmolten inorganic salts are members selected from the group consisting ofnitrates, nitrites, carbonates, sulfates, phosphates, and halides ofpotassium, sodium and lithium.
 14. A method in accordance with claim 1in which said temperature of step (c) is from about 100° C. to about350° C.
 15. A method in accordance with claim 1 in which saidrubber-forming substance is a member selected from the group consistingof polychloroprene and styrene-butadiene rubber, and said temperature ofstep (c) is from about 150° C. to about 300° C.
 16. A method inaccordance with claim 1 in which said rubber-forming substance isnatural rubber, and said temperature of step (c) is from about 150° C.to about 235° C.
 17. A method in accordance with claim 1 in which saidvulcanizing agent is a member selected from the group consisting oforganic peroxides, sulfur-containing compounds, selenium-containingcompounds, and tellurium-containing compounds.
 18. A method inaccordance with claim 1 in which said vulcanizing agent is a memberselected from the group consisting of diacyl peroxides, peroxyketals,dialkyl peroxides, mercaptothiazoles, thiuram sulfides, thiuramdisulfides, guanidines, zinc dialkyl dithiocarbamates, selecium dialkyldithiocarbamates, sodium diethyldithiocarbamate, potassiumdiethyldithiocarbamate, alkyl phenol sulfides, sulfur-containingpolymers, and benzothiazyl disulfide.
 19. A method in accordance withclaim 1 in which said vulcanizing agent is an organic peroxide.
 20. Amethod in accordance with claim 1 in which said vulcanizing agent is acombination of an organic peroxide and a member selected from the groupconsisting of multifunctional salts of acrylic and methacrylic acids.21. A method in accordance with claim 1 in which said vulcanizing agentis a dicumyl peroxide.
 22. A method in accordance with claim 1 in whichsaid vulcanizing agent is a combination of dicumyl peroxide and zincdimethacrylate.
 23. A method in accordance with claim 1 in which saidrubber-forming substance of step (a) is not vulcanized prior to step(a).
 24. A method in accordance with claim 1 further comprisingpartially vulcanizing said rubber-forming substance prior to step (a).25. A method in accordance with claim 24 in which said partialvulcanizing is achieved by high energy irradiation.
 26. A method forincreasing the tensile strength of an article of vulcanized rubber, saidmethod comprising: (a) immersing said article in a solution of avulcanizing agent to cause said article to absorb said secondvulcanizing agent from said solution; (b) immersing said articlecontaining said absorbed vulcanizing agent in a chemically inert liquidbath at a temperature and for a period of time sufficient to causefurther vulcanization said vulcanized rubber by said vulcanizing agent;and (c) withdrawing said article from said liquid bath.
 27. A method inaccordance with claim 26 in which said vulcanized rubber is vulcanizedcis-1,4-polyisoprene.
 28. A method in accordance with claim 26 in whichsaid liquid bath is a member selected from the group consisting ofmolten inorganic salts and mixtures thereof.
 29. A method for thepreparation of a substantially pore-free article of rubber, said methodcomprising: (a) dipping a forming member in a liquid medium comprising(i) a rubber-forming substance and (ii) a first vulcanizing agent, saidforming member having an outer surface with a contour complementary tothat of said article; (b) withdrawing said forming member from saidliquid medium in such a manner as to leave a film of said liquid mediumover said outer surface; (c) immersing said forming member with saidliquid film thereon in a first chemically inert liquid bath at atemperature and for a period of time sufficient to cause vulcanizationof said rubber-forming substance by said first vulcanizing agent; (d)withdrawing said forming member with a film of vulcanized rubber thereonfrom said liquid bath; (e) immersing said film of vulcanized rubberformed in step (d) in a solution of a second vulcanizing agent to causesaid film to absorb said second vulcanizing agent from said solution;(f) immersing said film containing said absorbed second vulcanizingagent in a second chemically inert liquid bath at a temperature and fora period of time sufficient to cause further vulcanization of saidrubber-forming substance by said second vulcanizing agent; and (g)withdrawing said film from said second liquid bath to achieve saidsubstantially pore-free rubber article.
 30. A method in accordance withclaim 29 in which said rubber is cis-1,4-polyisoprene.
 31. A method inaccordance with claim 29 in which said liquid bath is a member selectedfrom the group consisting of molten inorganic salts and mixturesthereof.
 32. A dip-molded article of a rubber other thancis-1,4-polyisoprene that is substantially pore-free, formed by aprocess comprising: (a) dipping a forming member in a liquid mediumcomprising (i) a rubber-forming substance other thancis-1,4-polyisoprene and (ii) a vulcanizing agent, said forming memberhaving an outer surface with a contour complementary to that of saidarticle; (b) withdrawing said forming member from said liquid medium insuch a manner as to leave a film of said liquid medium over said outersurface; (c) immersing said forming member with said liquid film thereonin a chemically inert liquid bath at a temperature and for a period oftime sufficient to cause vulcanization of said rubber-forming substanceby said vulcanizing agent; and (d) withdrawing said forming member fromsaid liquid bath and separating said substantially pore-free article ofrubber article from said forming member.
 33. A dip-molded article inaccordance with claim 32 in which said rubber-forming substance is amember selected from the group consisting of natural rubber,polychloroprene, nitrile rubber, polyurethane, styrene block polymer,and butyl rubber.
 34. A dip-molded article in accordance with claim 32in which said rubber-forming substance is a polyurethane thermoplasticelastomer.
 35. A dip-molded article in accordance with claim 32 in whichsaid rubber-forming substance is a polyurethane thermoplastic elastomerand said liquid medium is an aqueous dispersion.
 36. A dip-moldedarticle in accordance with claim 32 in which said rubber-formingsubstance is a thermoplastic styrenic block copolymer.
 37. A dip-moldedarticle in accordance with claim 32 in which said rubber-formingsubstance is a thermoplastic styrenic block copolymer and said liquidmedium is an aqueous dispersion.
 38. A dip-molded article in accordancewith claim 32 in which said liquid medium of step (a) is a latex.
 39. Adip-molded article in accordance with claim 32 in which said liquidmedium of step (a) is a solution.
 40. A dip-molded article in accordancewith claim 32 in which said rubber-forming substance is a memberselected from the group consisting of natural rubber andpolychloroprene.
 41. A dip-molded article in accordance with claim 32 inwhich said liquid bath of step (c) is a member selected from the groupconsisting of molten inorganic salts, oils, glycols, liquified metals,and brine solutions.
 42. A dip-molded article in accordance with claim32 in which said liquid bath of step (c) is a member selected from thegroup consisting of molten inorganic salts, silicone oils, and glycols.43. A dip-molded article in accordance with claim 32 in which saidliquid bath of step (c) is a member selected from the group consistingof molten inorganic salts and mixtures thereof.
 44. A dip-molded articlein accordance with claim 43 in which said molten inorganic salts aremembers selected from the group consisting of nitrates, nitrites,carbonates, sulfates, phosphates, and halides of potassium, sodium andlithium.
 45. A dip-molded article in accordance with claim 32 in whichsaid temperature of step (c) is from about 100° C. to about 350° C. 46.A dip-molded article in accordance with claim 32 in which saidrubber-forming substance is a member selected from the group consistingof polychloroprene and styrene-butadiene rubber, and said temperature ofstep (c) is from about 150° C. to about 300° C.
 47. A dip-molded articlein accordance with claim 32 in which said rubber-forming substance isnatural rubber, and said temperature of step (c) is from about 150° C.to about 235° C.
 48. A dip-molded article in accordance with claim 32 inwhich said vulcanizing agent is a member selected from the groupconsisting of organic peroxides, sulfur-containing compounds,selenium-containing compounds, and tellurium-containing compounds.
 49. Adip-molded article in accordance with claim 32 in which said vulcanizingagent is a member selected from the group consisting of diacylperoxides, peroxyketals, dialkyl peroxides, mercaptothiazoles, thiuramsulfides, thiuram disulfides, guanidines, zinc dialkyl dithiocarbamates,selecium dialkyl dithiocarbamates, sodium diethyldithiocarbamate,potassium diethyldithiocarbamate, alkyl phenol sulfides,sulfur-containing polymers, and benzothiazyl disulfide.
 50. A dip-moldedarticle in accordance with claim 32 in which said vulcanizing agent isan organic peroxide.
 51. A dip-molded article in accordance with claim32 in which said vulcanizing agent is dicumyl peroxide.
 52. A dip-moldedarticle in accordance with claim 32 in which said rubber-formingsubstance of step (a) is not vulcanized prior to step (a)
 53. Adip-molded article in accordance with claim 32 in which saidrubber-forming substance is partially vulcanized prior to step (a). 54.A dip-molded article in accordance with claim 53 in which saidrubber-forming substance is partially vulcanized prior to step (a) byhigh energy irradiation.